Zinc oxide varistors are employed in voltage surge arrester devices for shunting surge currents while maintaining the ability to operate under line voltage conditions. These varistors have a high exponent "n" in the voltage-current relationship I=KV.sup.n for a varistor, where I is the current through the varistors, K is a constant and V is the voltage across the varistor. High exponent zinc oxide compound varistors can have sufficient resistance at normal line voltage to limit the current through the varistor to a low value, but resistance at high currents is low so that the varistor voltage with surge current flowing is held to a level low enough to prevent damage to the insulation of the equipment being protected by the varistor.
Because the varistors are continuously connected from line-to-ground a continuous current flows through the varistor, and the current causes a small amount of power to be dissipated by the varistors at normal system voltage and at normal operating temperature. The magnitude of both the current and the resulting power dissipation increases as the varistor temperature increases. Some means must therefore be provided to remove heat from the varistor to prevent thermal runaway. The means must not only be capable of preventing thermal runaway under normal conditions, but it must also be capable of dissipating the heat resulting from high current surges. One effective means for removing the heat from the varistor bodies employs an aluminum oxide filled silicone resin. Each individual varistor disk is cast within a thick quantity of the resin material prior to insertion within the surge arrester housing. The thick silicone material carries heat away from the varistor to the walls of the surge arrester body. The use of a silicone encapsulant as a heat transfer means in zinc oxide varistors is described within U.S. Pat. No. 4,092,694 and 4,100,588.
The process of silicone encapsulation is extremely difficult to implement in a high production operation. Varistor disks are encapsulated within the silicone by means of a molding operation and individual varistor disks or a pair of disks must be inserted within a separate mold before the silicone encapsulant is added. After a sufficient quantity of time has lapsed for the silicone material to cure, the encapsulated disks must then be manually removed from the molds. The high material costs for the quantity of silicone material employed as well as the custom mold forming operation have made the use of zinc oxide varistors in surge arrester devices very expensive. One of the purposes of this invention is to provide zinc oxide varistors with an improved heat sink thermal transfer system at a greatly reduced manufacturing cost.
U.S. Pat. No. 2,870,307 (Milliken et al) describes an early method of providing a weatherproof resistor which is cooled by means of a plurality of thin plates made from aluminum or copper. Cooling air is freely circulated about the periphery of the plates during operation of the resistor.
The instant invention differs substantially from the device described within the aforementioned patent for the reason that the amount of heat required to be rapidly transferred away from a zinc oxide varistor disk is too large to be carried by the aluminum or copper plates. The mechanism of heat transfer required by the resistors described within the aforementioned patent is by the mechanism of radiation and convection into the surrounding air and is in effect, a steady state heat transfer system. The heat transfer mechanism proposed within the instant invention is a "heat sink" which rapidly absorbs heat and is, in effect, a transient responding system. The heat sink elements proposed within the instant invention can therefore have a diameter corresponding to the zinc oxide varistors employed whereas the radiating plates described within the patent to Milliken et al should be larger than the resistors to be cooled to provide a large radiating surface.